A new trinor-guaiane sesquiterpene has been isolated from an Indonesian soft coral Anthelia sp. The planar structure, possessing an octahydroazulene skeleton, and relative stereochemistry were established by analyzing 1D and 2D NMR data, including NOB experiments. Its absolute stereochemistry was elucidated to be IS, 48, 7R, and lOR by comparing observed and calculated optical rotation values. The new compound showed weak cytotoxicity against NBT-T2 cells.